With the miniaturization and integration of semiconductor devices, the miniaturization of a circuit pattern is going on in the management or development of the manufacturing process, so that the size of a hole or a groove formed on the wafer is about 10 nm in the case of a fine hole or groove. For this reason, demands for measuring a fine pattern having a size of tens of nanometers with high accuracy and at high speed are increasing more and more. In order to meet the demands of nano-resolution observation that cannot be addressed by the optical microscope, the probe diameter of the electron beam of a scanning electron microscope (hereinafter, referred to as an SEM) has been reduced year by year to about nm. This SEM is a high-resolution observation image acquisition means next to a scanning probe microscope (hereinafter, referred to as an SPM) having atomic resolution.
However, restrictions on the use represented by the damage to a sample are an obstacle to expanding its applicability. For example, a critical dimension scanning electron microscope (hereinafter, referred to as a CD-SEM) is an essential measuring apparatus in semiconductor lithography management, but the damage to the resist by the electron beam is an obstacle to measurement. As effective means for reducing this damage, measurement at 100 eV or less has been proposed. Thus, electron beam of ultra-low acceleration voltage is required for low damage observation of soft materials. However, it is not possible to obtain the desired resolution with the electron beam of ultra-low acceleration voltage since optical aberration and a diffraction aberration are increased. The optical aberration correction is an aberration caused geometrically and optically by a lens or the like of the electron optical system, and has a feature that the aberration increases as the opening angle of the beam on a sample increases. On the other hand, the diffraction aberration is an aberration caused by the wave nature of electrons, and has a feature that the aberration increases as the opening angle of the beam on a sample decreases. Assuming that the allowable range of the sample height at which the resolution is degraded by 10% is the depth of focus of an observation image, the depth of focus increases as the beam opening angle on the sample decreases.
As a basic principle of an optical aberration correction method, NPL 1 discloses a method of correcting a chromatic aberration, among optical aberrations, by creating the straight-through condition of Wien filters by multiple poles when the electric field and the magnetic field are superimposed.
As a basic principle of annular illumination, PTL 1 discloses a scanning charged particle beam microscope characterized in that a passage opening to restrict the passage of a charged particle beam is disposed between a charged particle source and a scanning deflector and the passage opening includes a member, which restricts the passage of a charged particle beam, at the center of the opening.
In order to efficiently obtain the multivalent ion beam of the microwave ion source, PTL 2 discloses a method for realizing stable beam extraction by arranging a solenoid coil and a magnet array in the ion source plasma chamber and confining the plasma so that the magnetic field strength is minimum in a central plasma portion on the average in both the axial direction and the radial direction when the combined magnetic field shape is seen from the plasma side.
In order to perform adjustment easily with little change of the electron orbit by reducing the power of the energy filter of the electron microscope, PTL 3 discloses a magnet in which auxiliary coils for adjusting the balance, which are independent of and adjacent to main coils, are wound in a solenoid shape with pole pieces of the upper and lower portions interposed therebetween and which generates a magnetic field in a pole piece gap.
In addition to the SEM, a transmission electron microscope (hereinafter, referred to as a TEM), a scanning transmission electron microscope (hereinafter, referred to as an STEM), and the like are included in apparatuses that perform observation, measurement, and inspection using a charged particle beam. As a common feature of structures of these optical systems, the spherical aberration and the chromatic aberration have been dominant factors in the resolution degradation. That is, since the spherical aberration has been a dominant factor rather than the chromatic aberration in the STEM or the TEM and the chromatic aberration has been a dominant factor rather than the spherical aberration in the SEM, optical aberration correction techniques optimized for the respective microscopes have been developed. By the appearance of these optical aberration correction techniques, aberrations of the entire optical system are suppressed, and a diffraction aberration is now a dominant factor in the resolution degradation as a common feature of the structures of the optical systems.